In general, electrochemical cells are composed of a negative electrode, a positive electrode and an electrolyte enabling charge carriers to transfer from one electrode to the other.
Electrochemical cells of the metal-air type are generally composed of a liquid electrolyte. The negative electrode, typically formed from a metal compound M, breaks down into Mn+ions during discharge while air is reduced at the positive electrode, called the air electrode, according to the reactions:
Discharge at the negative electrode: M→Mn++n e−
Discharge at the positive electrode: O2+2 H2O+4 e−→4 OH−
Alkali metals from group 1 of the periodic table used as active material for the negative electrode are generally not stable in the aqueous electrolyte, and the electrode made of such an alkaline material must be protected by a waterproof barrier.
To ensure such protection, a rigid ceramic membrane is commonly used. Conductive ceramics ensure the dual function of enabling the passage of ions from the negative electrode to the liquid electrolyte and of preventing the liquid electrolyte from directly entering into contact with the metal of the electrode.
However, using rigid ceramic membranes involves a double constraint the thickness of the membrane. This thickness must be sufficiently thick to guarantee good mechanical strength due to the fragility of the ceramic, but also sufficiently thin to reduce the ionic resistance of the ceramic and to limit power losses associated with this resistance. This compromise between ionic conductivity and mechanical strength limits the performance of these membranes.
When the cell is recharged, oxygen is produced at the positive electrode and the metal is deposited by reduction at the negative electrode, according to the reactions:
Recharge at the negative electrode: Mn++n e−→M
Recharge at the positive electrode: 4 OH−→O2+2 H2O+4 e−
This then poses a second problem affecting the negative electrodes protected by a rigid ceramic membrane. Indeed, metal is generally not deposited homogenously on the negative electrode. Consequently, it is not uncommon for the negative electrode to undergo significant structural modifications after several charge/discharge cycles. In practice, such structural modifications result in the formation of cavities and protuberances known as dendrites on the surface of the negative electrode.
When ions traverse the ceramic membrane from the electrolyte to the electrode, they do not always deposit uniformly on the surface of the negative electrode. Therefore mechanical stresses at the interface between the protective ceramic membrane and the negative electrode are created. In addition, eventually some areas of the negative electrode are no longer in direct contact with the ceramic membrane, thereby reducing the contact surface between the membrane and the electrode. This further reduces the conduction area of ions through the membrane and creates non-active areas on the electrode where contact between the membrane and the electrode is lost.
Producing rigid ceramic membranes presents another disadvantage connected to the fact that they are only suitable for a single size and shape of electrode. The method for producing a ceramic membrane should be suitable for matching the geometry of a given electrode, and does not enable membranes adaptable to any type of electrode to be produced.
For the reasons mentioned above, a means for protecting a metal electrode of an electrochemical cell that enables ions to be effectively conducted between the electrode and the electrolyte, while protecting the metal electrode from water, is being sought.